1. Field of the Invention
The present invention relates to a displacement measuring apparatus by using a radiation beam, such as an optical linear encoder or an optical rotary encoder.
2. Related Background Art
As shown in U.S. Pat. No. 3,726,595, U.S. Pat. No. 4,629,886 or U.S. Pat. No. 4,676,645, a measuring apparatus of the displacement of a body to be detected to which a linear scale is attached has been known, where a laser light is impinged onto the linear scale and a reflected light from the scale is detected by a photo-detector.
In the displacement measuring apparatus disclosed in the above-mentioned U.S. Patents, the scale is formed with diffraction gratings, an interference fringe is formed with a diffraction light emitted from the scale onto which a laser light is radiated so as to detect displacement of a body to be detected or the scale on the basis of a signal obtained by the photo-electric conversion of the interference fringe. Thus the apparatus is capable of obtaining a very high resolving power measurement. However, the laser light may leak from the apparatus outside when the linear scale moves more than an effective length (i.e., the length of the diffraction gratings accompanied with a movement of the body to be detected, or the scale inclines more than a certain angle against a displacement direction of the scale, or the scale deviates to an orthogonal direction with respect to the displacement direction.